A light emitting diode (LED) is one kind of element that converts electrical energy to light energy by utilizing the characteristics of semiconductors. Because LEDs have a good energy conversion efficiency and a long life, they are becoming more and more popular in applications for various kinds of electronic equipment such as lighting devices, illuminations, and displays. This is demanding in recent years that light emitting elements used for LEDs have even higher brightness.
Light emitting elements used for such LEDs have a structure in which an n-type semiconductor layer such as an n-type GaN layer, a light emitting layer such as an InGaN layer, and a p-type semiconductor layer such as a p-type GaN layer are formed in this order on a substrate, and electrons injected from the n-type semiconductor layer and positive holes injected from the p-type semiconductor layer are recombined in the light emitting layer, whereby light is emitted.
With light emitting elements having such a structure, a technology is known in which a GaN-based semiconductor layer is formed using an epitaxial growth method for growing a GaN-based semiconductor crystal on a substrate crystal such that the semiconductor crystal is oriented in the same arrangement as the crystalline face of the substrate base material. As substrates for crystal growth, single crystal sapphire substrates having excellent characteristics in terms of mechanical and thermal characteristics, chemical stability, and optical transparency are used in many cases.
There is a problem, however, in that the crystal growth of a GaN layer on a single crystal sapphire substrate disturbs the GaN crystal orientation during the growth and causes defects because there is a difference between the crystal lattice constant of sapphire and that of GaN. There is also a problem in that the difference between the refractive index of the sapphire substrate and that of the GaN-based semiconductor layer causes light emitted from the light emitting layer to be totally reflected at the interface between the sapphire layer and the GaN-based semiconductor layer, whereby the light is confined in the GaN-based semiconductor layer. These problems pose a challenge in that light to be taken outwardly is reduced.
As a method of enhancing the efficiency of taking out light from the light emitting layer, a method is known in which a resist pattern is formed on a sapphire substrate, and the pattern is used as a mask to carry out dry etching on the sapphire substrate, whereby a convex pattern is formed on the surface of the sapphire substrate (see, for example, Patent Document 1 to 3). The formation of a convex pattern promotes crystal dislocation along with the convex pattern when a GaN-based semiconductor layer is formed, and thus reduces the defects of the GaN layer when compared with the use of a smooth sapphire substrate. In addition, scattering and diffracting light emitted from the light emitting layer enables suppression of total reflection at the layer interface.